Nowadays predominantly lithographic exposure methods are used for producing semiconductor components, e.g. computer chips, and other finely structured components. In this case, use is made of masks (reticles) or other patterning devices which carry or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a lithography exposure apparatus between an illumination system and a projection objective in the region of an object plane of the projection objective and illuminated with an illumination radiation provided by the illumination system. The radiation altered by the pattern passes as projection radiation through the projection objective, which images the pattern onto a substrate to be exposed. A surface of the substrate is positioned in an image plane of the projection objective, which is optically conjugated to the object plane. In general, the substrate surface is coated with a radiation-sensitive layer (resist).
In order to be able to produce even finer structures, various approaches are pursued. By way of example, the resolution capability of a projection objective can be increased by enlarging the image-side numerical aperture of the projection objective. Another approach consists in employing shorter wavelengths of the electromagnetic radiation. For example, optical systems have been developed which use electromagnetic radiation from the extreme ultraviolet range (EUV), in particular having operating wavelengths in the range of between 5 nanometer (nm) and 30 nm.
Radiation from the EUV range cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at longer wavelengths. Therefore, mirror systems are used for EUV lithography. Not only in optical systems used for EUV lithography, but also in optical systems for lithography using light from the deep ultraviolet range (DUV, operating wavelength below 300 nm) or vacuum ultraviolet range (VUV, operating wavelength below 200 nm) mirrors are applied.
In order to ensure a best possible quality of the lithographic imaging, it is generally endeavored to produce a well defined intensity distribution in the illumination field illuminated by the illumination system and to image the pattern of the mask with as little as possible aberrations into the image field. These requirements not only have to be met by an optical system at the time of its delivery, but have to be maintained over the entire lifetime of the optical system without significant change. While in the former case possible deviations are substantially based on design residues and manufacturing faults, changes over the lifetime are often substantially caused by ageing phenomena. One source for deviations is a possible deviation of a mirror surface shape from the specified surface shape, possibly causing relative phase changes within light reflected by the mirror.
Many modern lithography processes use resolution enhancing techniques, such as double exposure, multiple exposure or spacering. These techniques allow generating fine structures through a sequence of subsequent exposure steps. These techniques require that structures generated in subsequent steps should be superimposed with high superimposition accuracy of successive exposures. As a consequence, requirements based on exact lateral image positioning are increased when compared to single exposure techniques.
Further, exact positioning of a focus plane becomes more difficult as the image side numerical aperture NA is increased and wavelengths are reduced. For example, the range of depth of focus is proportional to the wavelength and inversely proportional to the square of the image side numerical aperture. EUV wavelengths at numerical apertures NA larger than 0.4 or wavelengths in the deep ultraviolet range at numerical apertures above 1.1, for example, will reduce the range of depth of focus to values which may be less than 70 nm, for example. As a consequence, the axial position of the focus plane needs to be controlled very precisely.
Further, telecentricity effects may influence the exact positioning of an image relative to the desired image position. Telecentricity may cause the image position to be inclined relative to the propagation direction of the radiation. Therefore, a change in axial position of the exposure area may cause a lateral displacement of the actual image from the desired image position, thereby influencing the lateral superimposition accuracy in a negative way.
In summary, the position of focus in axial as well as lateral position should be controlled very carefully. Further, it should be considered that a real substrate surface, usually coated with a light sensitive coating, may not necessarily be a planar surface, but instead may deviate from planarity. This may require adjustment of focus position between exposure steps or even during a single scanning exposure operation. Further, it would be desirable to be able to control precisely not only the position of focus, but also other factors influencing image quality, such as the imaging scale or aberrations like distortion.
Higher aberrations may be caused by heating of optical elements, which may cause variation of refractive index and/or deformation of optical surfaces. Gravitational forces may also influence the optical effect of optical elements in that these optical surfaces may be deformed due to gravity. Further, aging processes cannot be excluded, which may contribute to aberrations.
Dynamic manipulator systems are frequently employed to account for time-dependent influences on the quality of the lithographic process.
WO 2012/126954 A1 discloses a mirror arrangement for correcting the illumination intensity distribution and the illumination angle distribution in the illumination field of an EUV illumination system. The mirror arrangement comprises a plurality of mirror elements forming a mirror surface, wherein each mirror element has a substrate, on which is applied a multilayer arrangement having a reflective effect with respect to radiation from the EUV range. Each multilayer arrangement includes a piezoelectric layer having a thickness which can be controlled by an electric field generated by an associated electrode arrangement. It is thereby possible for the piezoelectric layers of the mirror elements to be controlled independently of one another and thus to be adjusted with regard to their layer thicknesses individually. As a consequence thereof, the reflection properties of the mirror arrangement can be influenced locally differently over the mirror surface, allowing the correction of the illumination intensity distribution and the illumination angle distribution.